1. Field of the Invention
Embodiments of the present invention relate to a charging device configured to deposit charge on a surface of a latent-image bearer by establishing an electric discharge between a discharge electrode, to which a charging bias is applied, and the latent-image bearer through a grid electrode, to which a grid bias is applied. Some embodiments relate to an image forming method and an image forming apparatus such as a copier, a facsimile, or a printer configured to deposit charge on a latent-image bearer, which is a to-be-charged member, using the charging device.
2. Description of the Related Art
Known examples of this type of image forming apparatus include an image forming apparatus disclosed in Japanese Laid-open Patent Application No. H4-163565. The image forming apparatus deposits charge uniformly on a surface of a photoconductor, which is a latent-image bearer, using a charging device and thereafter optically writes an electrostatic latent image to the charged surface of the photoconductor using an optical scanning system. The image forming apparatus obtains a toner image by developing the electrostatic latent image with toner and transfers the toner image from the photoconductor onto a recording sheet. The charging device includes a corona electrode, which is a discharge electrode, and a grid electrode arranged between the corona electrode and the photoconductor. The charging device further includes a corona power supply, which is a charging power supply for feeding a charging bias to be applied to the corona electrode, and a grid power supply for feeding a grid bias to be applied to the lattice-like grid electrode with constant voltage control. The charging device establishes a discharge between the corona electrode and the photoconductor through the grid electrode, thereby depositing charge uniformly on the surface of the photoconductor. During an initial operation at power ON, a control unit performs a determining process of determining an output setpoint value for the constant voltage control of the grid bias in the following manner to deposit charge on the surface of the photoconductor to a desired potential over a long period. More specifically, an amount of electric current flowing from the corona electrode to the photoconductor is detected. The output setpoint value for the constant voltage control of the grid bias, which allows charging the photoconductor to the desired potential, is determined based on a result of the detection. The determined output setpoint value is used in subsequent charging.
The reason why the output setpoint value for the constant voltage control of the grid bias is determined in this manner is as follows. That is, the longer the photoconductor is used, the more a surface layer of the photoconductor is worn, causing charging performance of the photoconductor to deteriorate. Accordingly, with a configuration which simply maintains the output setpoint value for the constant voltage control of the grid bias constant, the charging potential of the photoconductor drops with time as the surface layer of the photoconductor is worn and adversely affects an image. In contrast, with a configuration in which, as in the image forming apparatus disclosed in Japanese Laid-open Patent Application No. H4-163565, the determining process described above is performed at power ON, the photoconductor can be charged to a desired charging potential even when the surface layer of the photoconductor is worn to some degree.
Meanwhile, Japanese Laid-open Patent Application No. 2010-181737 discloses an image forming apparatus configured as follows. The image forming apparatus causes, as does the image forming apparatus disclosed in Japanese Laid-open Patent Application No. H4-163565, a grid power supply to feed a grid bias with constant voltage control and a corona power supply to feed a charging bias to be applied to a corona electrode with constant current control. However, in contrast to the image forming apparatus disclosed in Japanese Laid-open Patent Application No. H4-163565, the image forming apparatus disclosed in Japanese Laid-open Patent Application No. 2010-181737 maintains an output setpoint value for the constant voltage control of the grid bias constant over a long period. The image forming apparatus increases an output setpoint value for the constant current control in such a manner that the output setpoint value increases with an increase in a measurement result of a physical quantity which is proportional to the thickness of a surface layer of a photoconductor. Examples of the physical quantity include a cumulative number of revolutions of the photoconductor.
With this configuration, it is expected that the image forming apparatus disclosed in Japanese Laid-open Patent Application No. 2010-181737 can deposit charge on the photoconductor to a desired charging potential stably while avoiding generation of unnecessary ozone and waste of energy in contrast to the image forming apparatus disclosed in Japanese Laid-open Patent Application No. H4-163565. More specifically, after the surface layer of the photoconductor is worn to a certain degree, even if the output setpoint value for the constant voltage control of the grid bias is adjusted, the charging potential cannot be increased to a target value unless a relatively large amount of electric current is supplied to the corona electrode. The image forming apparatus disclosed in Japanese Laid-open Patent Application No. H4-163565 requires that the output setpoint value for the constant voltage control of the grid bias be set to a relatively large value so that the photoconductor can be charged to the target charging potential even if the surface layer of the photoconductor is worn near to the end of its usable life. However, if the output setpoint value is set to such a value, in a state where the surface layer of the photoconductor is hardly worn and therefore the photoconductor delivers sufficient charging performance, a charging current larger than a necessary amount is fed to the photoconductor, resulting in generation of unnecessary ozone and waste of energy. In contrast, the image forming apparatus disclosed in Japanese Laid-open Patent Application No. 2010-181737 is capable of setting the output setpoint value for the constant current control of the charging bias to an appropriate value depending on the degree of wear of the surface layer of the photoconductor. Accordingly, the image forming apparatus is capable of depositing charge on the photoconductor to a desired charging potential while avoiding generation of unnecessary ozone and waste of energy when the surface layer of the photoconductor is not worn severely.
However, the image forming apparatus disclosed in Japanese Laid-open Patent Application No. 2010-181737 is disadvantageously prone to insufficient image density in a low-temperature and low-humidity environment. More specifically, when an environment changes to a low-temperature and low-humidity environment, electrical resistance of the surface layer of the photoconductor increases, and the potential of an electrostatic latent image formed by optical writing onto the photoconductor is increased to be higher than that in a normal-temperature and normal-humidity environment. As a result, a developing potential, which is the difference between a potential at a developer bearer such as a developing roller and a potential at the electrostatic latent image, drops, resulting in insufficient image density.
Under the circumstances, there is a need for a charging device described below, an image forming apparatus including the charging device and an image forming method. The charging device is capable of depositing charge on an electrostatic latent bearer to a desired charging potential while avoiding generation of unnecessary ozone and waste of energy and, simultaneously, obtaining stable image density independently of an environment.
It is an object of the present invention to at least partially solve the problems in the conventional technology.